I bet this question was asked before, but the search didn't spit out anything...
What do you think is under Shy Guys mask?
My theory: It's a shy Toadie without the ability to fly. They wear the same clothes as Shy Guys, just without the belt.
Yeah, I know, in Luigi's Mansion there wasn't anything underneath the mask except of two glowing eyes but they were just ghosts, weren't they?
On the other hand, it must be something scary, because of the trophy-sequence in Mario Power Tennis... But isn't Luigi afraid of everything? XD"